Accuracy in vehicle engine control parameters has become increasingly important in reducing vehicle emissions and improving economy. One of the parameters significantly affecting emissions and economy is the timing of combustion in the cylinders of the vehicle engine. In a gas fueled engine, this timing involves the crankshaft angle location of spark. In a diesel fueled engine, the timing involves the crankshaft angle location of fuel injection.
In both gas and diesel engines, the crankshaft timing angles are referenced to the engine piston top dead center position which is the exact geometric position at which the motion of the piston in the engine cylinder reverses direction and at which the combustion chamber volume is at a minimum. Therefore, the accuracy of any control or diagnostic system for establishing or monitoring combustion timing can be no better than the accuracy of the location of the piston top dead center. It is apparent, therefore, that to accurately establish or monitor engine timing requires an accurate determination of the top dead center position of the pistons.
Numerous systems have been employed for providing an indication of the crankshaft angle at which a piston reaches a top dead center position. For example, one such system employs a once-per-revolution pickup on the engine flywheel which provides a reference engine angular position relative to top dead center. This pulse in conjunction with pulses provided at small angular increments of engine rotation provide for the detection of top dead center of each piston. Typically, the pulses provided at the angular increments of engine rotation are obtained by monitoring the passing of the teeth on the engine ring gear by an electromagnetic pickup. These teeth are spaced at two to three degree intervals around the ring gear so that an electrical pulse is provided for each two to three degrees of angular rotation of the engine. The reference pulse is typically provided by a single tooth monitored by an electromagnetic pickup whose output is a signal representative of the reference engine angular position.
By sensing the passing of the tooth representing the angular reference engine position and thereafter counting ring gear teeth, the system may continuously determine the top dead center position of each piston upon rotation of the engine. However, the accuracy of this type of system is limited by the accuracy in which the reference tooth was positioned on the engine flywheel relative to top dead center position. Further, in the case of a diesel engine wherein the reference pulse may be provided by a pickup located in the fuel pump, the accuracy of the reference pulse relative to engine top dead center position may vary considerably from the desired position thereby affecting the accuracy in the determination of engine top dead center position.